TLP

tlp is set of powersaving-scripts for Lenovo/IBM ThinkPads (c), which handle powersaving options according to the current power source. It was originally written by linrunner for Ubuntu.
A port is available at the AUR. Although it is written for ThinkPads, it should work on other laptops as well, but without ThinkPad specific functions. Some features are:

Kernel-Laptop-Mode and timouts for writeback of the filesystembuffer to hard disk

Advanced Power Management Level per hard disk

I/O scheduler per hard disk

SATA Aggressive Link Power Management (ALPM)

Wifi powersaving mode for wifi-cards with iwlagn driver

battery-charging thresholds on some ThinkPads with (optionally) tp_smapi

Wifi

Some intel wifi cards require to set their powersaving options after establishing connection. Therefore the original version of tlp adds a script to Template:Filename, which itself calls (as root)

tlp wifi $IFACE

where $IFACE is the current Interface (e.g. wlan0). In Arch Linux, there is no such possibility by default. If you own a wifi card, which is not supported out of the box, please use the post-up methods of your preferred networking manager (like netcfg oder NetworkManager).

If tlp sets the powersaving options properly on ac-state changed, you do not need to change anything. Check this, by issuing the following command twice, before and after unplugging your ac adapter: